There is a way to apparently add calling plus to the unlocked S8.
LTE Test is under com.test.LTEfunctionality and Enhanced LTE Services is under Mobile on my device.
credit: https://forum.xda-developers.com/galaxy-s8/help/sprint-calling-plus-update-unlocked-s8-t3730053

You can use the standard 'call your phone number' to get your voicemail. I know Google voice / Youmail is a popular option for some but I've heard some people might have trouble using it if they enable wifi calling? Not sure.
Alternatively, you can always flash the Sprint firmware onto the unlocked version. Just be prepared there is a ton of bloatware.

You are forgetting Black Friday sales.
The LG Nexus 5X was $80 off on November 27th, 2015.
http://www.androidpolice.com/2015/11/27/updated-killer-black-friday-deal-nexus-5x-80-off-amazon-bh-google-store-299349/

Companies may sometimes have secret menu's or friends and family type sales. This was really no different. See Sprint's print from 2006 below where they opened SERO to general public:
http://a676.g.akamaitech.net/f/676/773/60m/images.delivery.net/cm50content/1529/09008101806249a7/FriendsAndFamily.jpg
Russ was also the Vice President of Strategy for Sprint and in his FAQ states that he was able to provide referrals through his blog. An email for users to use was also posted on the front page of the Sprint community website around the same time IIRC.